A method for transferring a useful layer 3 onto a supporting substrate 4, shown in FIG. 1, is known from the prior art. This method includes the following main steps:                a) the formation of a fragilization plane 2 through the implantation of light species into a first substrate 1 in such a way as to foini a useful layer 3 between this plane and a surface of the first substrate;        b) the application of the support 4 onto the surface of the first substrate 1 to form an assembly to be fractured 5, having two exposed sides S1, S2;        c) the thermal fragilization treatment of the assembly to be fractured 5; and        d) the initiation and self-sustained propagation of a fracture wave in the first substrate 1 along the fragilization plane 2.        
During this method, the implanted species cause the development of microcavities. The effect of the thermal fragilization treatment is to promote the coalescence and pressurization of these microcavities. Under the effect of this thermal treatment alone, or through additional external forces, the initiation and self-sustained propagation of a fracture wave enables the transfer of the useful layer 3 through detachment in the fragilization plane 2.
This method, described notably in the documents WO2005/043615 and WO2005/043616 and designated by the name of “SMART CUT®,” is useful, in particular, for the manufacture of silicon-on-insulator substrates. In this case, the first substrate 1 and the support 4 are formed from a silicon wafer, and one or the other of the first substrate 1 and the support 4 are surface-oxidized.
These silicon-on-insulator substrates must comply with very precise specifications. This applies, in particular, to the average thickness and uniformity of thickness of the useful layer 3. Compliance with these specifications is required for the correct operation of the semiconductor devices that will be formed in or on this useful layer 3.
In some cases, the architecture of these semiconductor devices requires the provision of silicon-on-insulator substrates having a very low average thickness of the useful layer 3, for example, less than 50 nm, or even less than 10 nm, and a uniformity of thickness that is highly constant on the surface of the substrate (the normalized diameter of which is typically 200 mm, 300 mm, and even 450 mm for the next generations). The expected uniformity of thickness may thus be in the order of at most 1%, corresponding to variation maxima typically ranging from +/−0.1 nm to +/−1 nm over the entire surface of the wafer.
It is customary, in terms of the SMART CUT® method, to apply complementary steps of finishing of the useful layer 3, such as etchings or surface-smoothing thermal treatments in order to seek to achieve the expected specification level.
The applicants in the case of the present disclosure have observed the presence, following the fracture step, of variations in the thickness of the useful layer 3 with a quite specific profile. These variations in thickness, in fact, appear in the form of a periodic pattern, the size of which is on the order of a nanometer, or even a half-nanometer, and the wavelength of which is on order of a millimeter, or even a centimeter. The periodic pattern may be apparent over the entire useful layer, or over a part only. This periodic pattern is thus visible on the thickness variation profile (in angstroms) along a diameter of a useful layer of a silicon-on-insulator wafer with a diameter of 300 mm obtained according to the SMART CUT® method of the prior art shown in FIG. 2 by a continuous line.
It is particularly difficult to correct this particular thickness non-uniformity profile using customary finishing techniques (etching, sacrificial oxidation, thermal softening treatment) as these techniques are ineffective in the wavelength range that these patterns have. Consequently, this periodic pattern contributes to the thickness non-uniformity of the useful layer 3 following the application of the finishing steps, which does not allow the required uniformity level to be achieved when the latter is significant.